As shown in FIG. 9, a conventional flow detection device 100, which is disclosed in a patent document 1, includes a housing 102 and a flow sensor 103. The housing 102 forms a bypass passage 101, which receives a part of an airflow passing through a duct. The flow sensor 103 is arranged in the bypass passage 101 to detect an airflow passing through the bypass passage 101.
As shown in FIG. 10, the flow sensor 103 is in a plate shape. The flow sensor 103 is arranged so that a front-to-rear direction of the flow sensor 103 is substantially along a passage-width direction of the bypass passage 101. That is, the direction from the front side of the flow sensor 103 to the rear side of the flow sensor 103 is substantially along the width direction of the bypass passage 101. In the present configuration, when airflow passes through the bypass passage 101 and reaches the flow sensor 103, the airflow is divided into a first flow F1, which passes on the front side of the flow sensor 103, and a second flow F2, which passes on the rear side of the flow sensor 103. Thereafter, the first flow F1 and the second flow F2 merge on the downstream side relative to the flow sensor 103. In the bypass passage 101, a bent portion 101a is equipped on the downstream side relative to the flow sensor 103 to bend the airflow therein. The first flow F1 and the second flow F2 merge in the bent portion 101 while turning its direction.    (Patent Document 1) Publication of International Patent Application No. 2006-501453
In such a configuration, the first flow F1 and the second flow F2 may not merge with each other smoothly on the downstream relative to the flow sensor 103. In such a case, an output of the flow sensor 103 may become unstable, and accuracy of flow detection may become insufficient.
It is noted that, the patent document 1 may disclose a step formed on the wall surface of a bypass passage to generate a swirl flow. Nevertheless, the object of the step in the patent document 1 is not to smoothen an airflow on the front side of the flow sensor and an airflow on the rear side of the flow sensor.